Fibroblast growth factor was purified for the first time from the hypophysis of cattle as a substance that stimulates the growth of a fibroblast line NIH3T3. Thereafter, analogous proteins were identified in various tissues, and a group of these substances forms a polypeptide family (FGF family). To date, 22 types of protein belonging to the FGF family have been identified in vertebrates. As the biological activities of these types of protein, not only the fibroblast-growth stimulating activity, but also a wide range of actions are known, such as growth of the mesoderm and the neuroectoderm, angiogenesis, and limb bud formation in the developmental stage. FGF family members vary in terms of expression sites and expression times of genes. The genes thereof are often expressed only at specific sites in the developmental stage or in adults. As genes encoding the receptors of FGF, at least 4 types are known: FGFR1, FGFR2, FGFR3, and FGFR4. In addition, it is known that in FGFR1, FGFR2, and FGFR3, receptor proteins differing in terms of their extracellular domains are independently present due to differences in splicing. Furthermore, it is known that heparin and heparan sulfate proteoglycan interact with FGF and FGF receptors, so as to regulate the action. Furthermore, there are many proteins belonging to the FGF family because of structural similarity, but their biological activities, their receptor-binding abilities, and the like remain almost unknown. Characteristics of the FGF family have been completed as reviews (Ornitz D. M. and Itoh N. Fibroblast growth factors. Genome biology 2: 3005. 1-3005. 12, 2001).
FGF-23 was cloned for the first time from a mouse by data base search utilizing its homology with FGF-15 and the PCR method, and then human FGF-23 was cloned utilizing its sequence homology with that of mouse FGF-23 (Yamashita T., Yoshioka M., and Itoh N. Identification of a Novel Fibroblast Growth Factor, FGF-23, Preferentially Expressed in the Ventrolateral Thalamic Nucleus of the Brain. Biochem. Biophy. Res. Commun. 277: 494-498, 2000). Subsequently, in research on autosomal dominant hypophosphatemic rickets/osteomalacia (hereinafter referred to as ADHR), missense mutations were characteristically discovered in the FGF-23 genes of ADHR patients while narrowing the region of mutant genes in ADHR patients and identifying responsible genes (The ADHR Consortium. Autosomal dominant hypophosphatemic rickets is associated with mutations in FGF-23. Nature Genet. 26: 345-348, 2000). This discovery has strongly suggested physiological importance of FGF-23 in vivo. However, the biological activities of FGF-23 have remained unknown. In the meantime, the biological activity of FGF-23 has been determined by research on tumor-induced osteomalacia. It has been thought that in this disease, a tumor responsible for the disease produces and secretes a humoral factor inducing the disease, and by the action of this factor, morbidity such as hypophosphatemia or osteomalacia is developed.
In search of this disease-inducing factor produced by such responsible tumor, FGF-23 has been cloned as a gene that is expressed at high levels in tumors. Furthermore, it has been shown that by the administration of this factor, hypophosphatemia and osteomalacia are reproduced (Shimada T., Mizutani S., Muto T., Yoneya T., Hino R., Takeda S, Takeuchi Y., Fujita T., Fukumoto S and Yamashita T., Cloning and characterization of FGF23 as a causative factor of tumor-induced osteomalacia. Proc Natl. Acad. Sci. 98: 6500-6505, 2001). This research has shown the involvement of FGF-23 in in vivo metabolic control relating to phosphorus and calcium, and suggested that FGF-23 acts as a systemic factor expressing its action while circulating in vivo. However, the in vivo concentration and metabolism required for the expression of the action of FGF-23 have not been shown, and the physiological role of FGF-23 remains largely unknown. Moreover, as a disease presenting conditions analogous in clinical findings, X-linked hypophosphatemic rickets is known. However, the involvement of FGF-23 in the morbidity has not been revealed. Except for ADHR and tumor-induced osteomalacia, there are no known diseases that have been proven to be associated with FGF-23.
The above tumor-induced osteomalacia is characterized by showing abnormally low levels of blood phosphorus and 1α,25-dihydroxyvitamin D (hereinafter referred to as 1,25D) along with tumorigenesis. It is accompanied by decreased muscle force, or osteomalacia, and may result in dysbasia or dysstasia. In most cases, the responsible tumor for this disease is a benign tumor derived from mesenchymal cells. Most responsible tumors are poor in growth ability, and notable increases are barely observed during follow-up. Furthermore, although the progression of morbidity is observed, detailed examination such as whole body scanning by MRI inspection is often required to find a responsible tumor. Accordingly, some cases where confirmed diagnosis is not given and a diagnosis of hypophosphatemia with unknown causes is made are suspected of being tumor-induced osteomalacia. Currently the only method that results in a confirmed diagnosis of tumor-induced osteomalacia is to confirm recovery from conditions of the disease by tumorectomy. This is because there are no methods for examining the cause and effect relationship between tumorigenesis and conditions of disease by clinical tests. In some cases, removal of non-responsible tumors that are completely independent from conditions of disease has been conducted. To improve such circumstances, development of a method for clinical tests whereby differential diagnosis can be made for tumor-induced osteomalacia has been expected.
The fact that FGF-23 has action controlling in vivo phosphorus metabolism has been discovered. Parathyroid hormones and 1,25D that have been known to have action controlling phosphorus metabolism play a more important role in controlling calcium metabolism rather than in phosphorus metabolism. No molecules mainly controlling phosphorus metabolism have been known, and FGF-23 is expected to have such activity. In the meantime, a close relationship between phosphorus metabolism and calcium metabolism is clinically known. In particular, in terms of calcification of bone tissues and pathological ectopic calcification, it is difficult to consider the two separately. Based on the facts that FGF-23 in an excessive state induces osteomalacia and FGF-23 has action lowering 1,25D, FGF-23 may be involved not only in phosphorus metabolism but also extensively in controlling calcification and bone metabolism. Moreover, some of diseases with abnormalities in organs controlling mineral metabolism consisting mainly of the intestinal tract, the kidney, and the bone tissues may be associated with the excessive accumulation and the deficiency state of FGF-23. Development of a method for testing in vivo concentrations of FGF-23 is also expected to result in an understanding of such diseases, and in precise treatment of the diseases.
Suppression or removal of FGF-23 in morbidity induced by excessive FGF-23 can be a therapeutic method for the disease. One possible method is the inhibition of ligand-receptor interaction using an antagonist for the receptor of FGF-23 or a substance binding to FGF-23, and another possible method involves the removal of FGF-23 using a substance binding to FGF-23. However, there are no known substances that selectively suppress or remove FGF-23 by the above-mentioned methods.